In geological exploration it is desirable to obtain information regarding the various formations and structures which exist beneath the Earth's surface. Accordingly, various techniques have been employed to identify subsurface structures, geological strata, and hydrocarbon reserves, etc., to determine density, porosity, and composition, etc. when searching for and/or developing hydrocarbon reserves.
For example, seismology has been used extensively for providing subsurface mapping, such as to produce two-dimensional (2D) or three-dimensional (3D) images. In implementing seismic data collection for subsurface mapping, a seismic source, typically producing waves in the five to sixty Hz range, introduces seismic energy into the subsurface and multiple seismic transducers collect seismic energy that is reflected and/or refracted by subsurface structure, features, etc. Although seismic data may be processed to provide a relatively good understanding of the subsurface down to a resolution of approximately ten meters (with typical velocities of rock), the technique is not without its disadvantages. For example, a very small amount of hydrocarbon (e.g., oil or gas) in a medium results in a very large change in the acoustic properties of that region, and thus seismic techniques often result in false indication of economic hydrocarbon volumes.
Various forms of well data, such as from well cuttings, well logging, well history, etc., have been used to provide subsurface mapping, such as may be limited to one-dimensional (1D) mapping in the case of the use of well cuttings and some well logging techniques, or as may provide 2D mapping in the case of some well logging techniques and well history. In implementing well data collection for subsurface mapping, data available from the drilling, completion, or operation of one or more boreholes penetrating the subsurface is collected and analyzed. Although such data may provide excellent information with respect to the subsurface actually penetrated by the borehole, the resulting mapping is typically limited in the extent, both vertically and horizontally, into which the subsurface may be mapped. For example, well cuttings typically only provide reliable mapping information for the area immediately adjacent to the borehole and well logging data often only provides reliable mapping information for an area approximately two to five meters beyond the borehole, and interpretation is required to extend mapping across the large area between wellbores. Although well history information may provide mapping information with respect to a reservoir in a general sense, detailed information such as boundaries and strata is generally not provided through well history information. Accordingly, an accurate understanding of a large area of the subsurface is currently not possible using such well data.
A more recently developed technique used for subsurface mapping is controlled-source electromagnetic (CSEM) surveying. CSEM surveying exploits the differences in resistivity of various subsurface media (e.g., rock formations, shale, sand, briny water, hydrocarbons, etc.) for providing subsurface mapping, such as to provide 2D or 3D images. Specific operational aspects, as an example, with respect to the use of CSEM data are provided in U.S. Pat. No. 6,603,313 to Srnka and World Intellectual Property Organization publication number WO 2004/083898 A1, the disclosures of which are hereby incorporated herein by reference.
In implementing CSEM data collection for subsurface mapping, an electromagnetic source, typically producing electromagnetic waves in the 0.01 Hz to 1 Hz range, introduces electromagnetic energy into the subsurface and multiple electromagnetic transducers collect electromagnetic energy that is reflected and/or refracted by subsurface structure, features, etc. Such electromagnetic data may be processed to identify regions of interest with respect to hydrocarbon reserves using the fact that hydrocarbon reservoirs are generally associated with higher resistivity levels than most sedimentary rocks present in the Earth's subsurface. Moreover, because the resistivity of subsurface media is not greatly affected by a small amount of hydrocarbon (e.g., oil or gas) in a medium, false hydrocarbon indicators from formations which contain small amounts of hydrocarbons but which do not include enough of the hydrocarbon (i.e. include enough to be an economically producible reservoir) can be avoided. However, due at least in part to the very low frequencies that are used in CSEM surveying, subsurface mapping provided by CSEM techniques often provides an understanding of the subsurface down to a resolution of approximately one hundred meters.
Although various techniques, such as the Sharp boundary inversion process (see e.g., Alumbaugh et al., “Two-Dimensional Inversion of Marine Electromagnetic Data Using Seismic Reflection Data as A Priori Information,” see internet domain agu.org/meetings/fm06/fm06-sessions, 2006 AGU meeting, 11-15 Dec. 2006); the disclosure of which is hereby incorporated herein by reference), have been developed for use in sharpening various regions of resistivity in resistivity images, these techniques have primarily resulted in only sharpening the edges of the “clouds” representing the regions of resistivity in the resistivity images. For example, although such techniques may provide a sharper boundary between regions of resistivity, the techniques do not remove the ambiguity of where within the cloud any particular feature which may have been the source of the resistivity cloud is actually disposed. Due to the low resolution of CSEM survey data, and the near proximity of different strata in an electromagnetic sense, the regions of resistivity provided by currently available electromagnetic subsurface mapping techniques are quite large and very indeterminate (i.e., individual structures and features are hidden in a resistivity cloud).